1. Field of the Invention
This invention relates to an apparatus for forming a solder-layer or solder-film on a printed-wiring board.
2. Description of the Related Art
In general, for connecting a wiring material, a lead terminal of an electronic part, or the like to a conductive pad (part of wiring pattern) formed on a surface of a printed-wiring board, the wiring material or the like is bonded to the conductive pad by soldering. For mounting an electronic part, a solder-film is preliminarily formed on the pad. The electronic part is placed on the pad, on which the solder-film is already formed, with the lead terminals, etc. correctly arranged. After the solder-film is fused in a high temperature atmosphere, it is cooled so that the electronic part is mounted on the printed-wiring board. The present invention relates to an apparatus for forming a solder-film on such a pad as just mentioned.
An amount of solder for bonding the electronic part onto the printed-wiring board should be limited to an amount suitable for connecting the lead of the electronic part to the pad in order to obtain a reliable connection, maintain a uniformed product quality, enhance the working efficiency, and so forth. For this purpose, solder balls are heretofore used. The solder balls are spherical balls which are each carefully dimensioned to have a diameter corresponding to the amount of solder which is to be fed onto each pad. Many solder balls having different diameters are prepared depending on the configurations of the pad, the lead of the part, etc.
According to a conventional technique, a solder ball is placed on a specific pad on the printed-wiring board using a pincette or forceps and then contacted with a soldering iron so that the solder ball on the pad is fused and hardened to form a solder-film on the pad. This work is all manually performed. With respect to another conventional technique, a plate-like guide mask having through-holes in that area corresponding to the pads on which the solder-films are to be formed are laid up on the printed-wiring board, and a solder ball is inserted into each and every through-hole by manually handling a pincette. Then, the printed-wiring board is left in a high temperature atmosphere so that the solder balls are fused and thereafter hardened to form a solder-film on each pad. The time reqired for leaving the printed-wiring board in the high temperature atmosphere is about an hour.
According to the above-mentioned conventional techniques, since the work for pinching the solder balls with the pincette and placing them on respective pads, and the work for contacting a soldering iron to the solder balls for fusing are all manually performed, the working efficiency is not good. Especially, since there is recently a requirement for designing a printed-wiring board with a very small dimension and arranging electronic parts in high density, the pads are being gradually made smaller and smaller and therefore the diameter of each solder ball is being gradually made smaller and smaller. This makes the above kind of manual work even more difficult. According to the second-mentioned conventional technique, since the entirety of the printed-wiring board must be put into the high temperature atmosphere even if a single place of a pad is the only spot on which the solder-film is required to be formed, it sometimes gives rise to a problem, among others, in that the printed-wiring board is deteriorated in characteristic by thermal stress.